Several types of Keggin structures are known. Representative Keggin structures having a central tetrahedral Al(O)4 core are illustrated in FIGS. 6A-6C. These structures correspond to so-called ε-Keggin (Al13), δ-Keggin (Al13), and an Al30 cluster. These and other Keggin structures are described in Casey et al., Reviews in Mineralogy & Geochemistry 44:167-190 (2001).
While these Keggin structures have potential practical applications, the synthesis of these structures limits their use. Synthesis of Keggin-like clusters typically involves lengthy reaction times and harsh reaction conditions. For example, synthesis of the Keggin-like Al cluster [Al13(μ3-OH)6(μ2-OH)18(H2O)24]Cl15 requires a 4-5 month synthesis/crystallization procedure as reported by Seichter et al., Eur. J. Inorg. Chem. (1998). Synthesis of the Keggin-like Al cluster [Al8(μ3-OH)2(μ2-OH)12(H2O)18](SO4)5 requires a 7 year synthesis/crystallization procedure as reported by Casey et al., Inorg. Chem. (2005). Synthesis of the Keggin-like Al cluster [Al15(μ3-O)4(μ3-OH)6(μ2-OH)14(hpdta)]3 requires an 8 day ligand-shell stabilized synthesis as reported in Schmitt et al., Angew. Chem. (2001). For convenience, diagrams of these Al(O)6 core materials are shown in FIGS. 7A-7C.
Because of the limitations of conventional synthesis, improved synthesis methods are needed, and new synthetic products made available by such improved synthesis methods.